Nanoparticles made of metals, semiconductors, or oxides are of interest for their mechanical, electrical, magnetic, optical, chemical and other properties. Noble metal nanostructures are of much interest because of their unique properties, including large optical field enhancements resulting in the strong scattering and absorption of light. The fields that are being impacted by the advancement in nanostructured materials include different areas such as electronics, materials, biology, medicine and other branches of physical sciences. To this end and as an example, gold nanoparticles are one of the most widely used classes of nanomaterials for chemical, bioanalytical, biomedical, optical and nanotechnological applications. While there are numerous methods known for the synthesis of gold nanoparticles, the ability to control the size, aspect ratio, shape and monodispersity of gold nanoparticles remains challenging and one of the main areas of research.
One important morphology of gold nanoparticles is rod- or cylindrical-shaped. Particles with this shape are called nanorods with typical dimensions ranging from 1-100 nm. Compared to other shapes of nanoparticles including spheres and shells, nanorods are more favorable for in-vivo applications due to their tunable optical resonance in the near infra-red region (NIR). Moreover, their relative scattering to absorption contribution can be easily tuned by a change in their dimensions. Gold nanorods offer superior NIR absorption and scattering at much smaller particle sizes. Smaller sized nanorods also offer better cell uptake as compared to the larger nanoshells and nanospheres. This, in addition to the potential noncytotoxicity of the gold material, easy optical tunability, and facile synthesis, makes gold nanorods promising nanoparticle agents for use in biomedical imaging and photothermal therapy applications.
Nanorods are often characterized by their aspect ratios, which is defined as the length to the width of a nanorod. A larger aspect ratio means a longer nanorod. Their width typically range from 5 nm to 30 nm and their length is about 20 nm or more. The state-of-the-art synthesis methods produce gold nanorods with aspect ratios less than 4.5 with no control over concentration of nanospheres [1, 2]. Gold nanorods with aspect ratios more than 4.5 are of interest due to their absorption in the NIR region of the spectrum.
Typical strategies for producing gold nanorods include: electrochemical methods [3], photochemical [4], and seed-based growth methods [1, 2]. In all these methods, gold nanorods are produced by reduction of a gold salt in the presence of a soft template in an aqueous solution. In electrochemical methods, a gold nanorod starts from a small gold nanoparticle and gradually grows to a rod-shaped object. Some limitations of this method are: a) high concentration of nanospheres compared to nanorods, b) lack of control over the length of the nanorods, and c) irreproducible results. In seed-based growth methods, 3-5 nm gold nanoparticles are used as seeds for growth of nanorods. The seed-mediated growth method was originally developed in laboratory test tubes for producing small amount of nanorods. Since its full development, this method has not been modified significantly and still small volumes of nanorods are prepared. New designs are necessary to overcome these limitations. As a mean to evolve this process towards more automated designs, a microfluidic flow process which contained micron size channels has also been used; however, this platform still has the weak points of the original growth method [5]. The disclosed invention overcomes the weak points and the inhomogeneities in nanorods growth present in the conventional approaches.